Capacitive silicon microphones are designed with a membrane and either one or two backplates separated by an air gap. Sound is transduced into electrical signals by detecting the varying capacitance between the membrane and the backplates as the membrane vibrates in response to sound waves. An electrical field is therefore required across the membrane and backplate electrodes. This electrical field is generally supplied by an Application Specific Integrated Circuit (ASIC) in the form of a bias voltage applied across the electrodes. The output of the ASIC is typically high-Ohmic in order to support a highly sensitive microphone with a high signal to noise ratio (SNR) and low current consumption. Consequently, the membrane and backplate electrodes must be well insulated from each other.
A reduction in insulation and resulting current leak between the electrodes can be caused by moisture, residue, or other particles and contamination that become connected between the electrodes. These current leaks can lead to increased noise, increased current consumption, and/or loss of sensitivity of the microphone system.
Many current microphone systems may place an insulating layer on the backplate facing the membrane in order to further insulate the electrodes from one another. This may accordingly prevent a current leak from this face of the backplate and the membrane. However, this cannot prevent a leakage from the opposite face of the backplate or from side walls of backplate perforation holes. Accordingly, these microphone systems may still be susceptible to current leaks and any resulting deteriorations in performance.
Alternatively, the membrane itself may be covered in one or more insulating layers, thereby potentially preventing current leaks. However, the placement of insulating layers on the membrane may impact the mechanical properties of the membrane, which should be sufficiently flexible in order to accurately capture sound wave vibrations. Consequently, this approach may lead to limited sensitivity of process-induced sensitivity variances. Furthermore, leakage between a back plate and other parts of the sensor may still result in the above described performance degradations.